Current nutritional strategies for very preterm infants in this country are resulting in postnatal growth failure from which the neonate does not recover by the time of hospital discharge. Undernutrition, particularly insufficient protein intake during critical development periods adversely affects long-term neurodevelopmental and health outcomes. Efforts to optimize nutritional support, therefore, particularly in the first weeks of life, are critical to reversing nutritionally related adverse outcomes in this vulnerable population. The only national published recommendation for providing comprehensive nutrition to the preterm neonate through the time of discharge is the American Academy of Pediatrics (AAP) recommendation to mimic the growth rate of the normal fetus of the same gestational age as the preterm jnfant. There are no guidelines by the AAP as to how this should be achieved, and no data in the literature to support or refute this recommendation. We believe that it is critical to determine if this is a valid recommendation. The overarching hypothesis to be addressed by this project is that protein accretion rates can be significantly improved in extremely low birth weight (ELBW) preterm infants during the critical nutritionally-sensitive early postnatal period. We believe that the proposed prospective studies of protein quantity, protein quality and energy intake on protein accretion will enhance the ability to optimize protein gain in these infants. Specific hypotheses to be addressed are: 1) that relatively high amino acid and energy intake initiated in the immediate neonatal period and sustained at fetal delivery rates through close monitoring of nutrient intake over the first 2 months of life will confer a prolonged anabolic advantage compared to current amino acid initiation and advancement strategies, and will determine whether or not fetal nutrient delivery rates administered to the preterm infant will achieve the AAP recommendation that extrauterine growth should approximate that of the fetus at the same gestational age;2) that optimal concentrations of essential amino acids in parenteral nutrition can be determined that will improve the quality of amino acid intake in order to maximize protein accretion rates;and 3) that energy intakes can be identified that will optimize protein accretion at any given amino acid intake.